FIGURE 1.

Overview of structural, H-bonding, and electronic changes in the fluorescence photocycle. The phenolic oxygen of the HBDI chromophore is H-bonded indirectly to glutamate 222 via a water molecule and serine 205. Coordinates are taken from PDB 1W7S (5). The electronically excited singlet state of the neutral and anion chromophore, A* and I* respectively, are indicated. ESPT reactions occur during the transition between the A* and I* states that have 10 and 75 ps time constant in D₂O (4,10). Blue excitation with 400 nm light is indicated to create the A* state.